When the FBI gets a credible tip about a terrorist nuclear weapon inside the U.S., the agency calls in the Department of Energy’s elite and secret Nuclear Emergency Search Team. Since its inception in 1975, NEST has been largely a volunteer organization. Most of its 1,000 or so scientists and engineers work at the national laboratories where America’s nuclear weapons are designed: Los Alamos, Sandia and Lawrence Livermore. Thanks to that experience, NEST volunteers know better than any law-enforcement agents how to identify and disable a nuclear device.

The volunteers, whose bags are always packed, can deploy by the hundreds on short notice. They work in concert with police and federal agents–always clandestinely, even during practice drills, to avoid public panic. “You won’t ever see our people walking along a city street or through a hotel or in Disney World,” says one former NEST volunteer.

The best way to locate a nuclear weapon is to search for its telltale radiation. That calls for sensitive detectors that can distinguish a bomb’s signature from legitimate sources of radioactivity. In most cases, searchers are looking for a lump of fissile material no larger than a soccer ball.

Many potential terrorist targets in the U.S.–including parts of major cities, football stadiums, convention centers and nuclear power plants–have already been “swept” by airborne detectors to establish baseline maps of naturally occurring radiation and innocent sources such as hospitals’ radioactive waste. Searchers can compare these maps with a fresh scan to check for anomalous sources of radioactivity.

Helicopters are particularly useful for this work because, as one nuclear weapons specialist explains, “ceilings and roofs are less dense than most building walls,” and helicopters can hover close to them. When an unusual or previously unaccounted-for source of radiation is detected from above, its location is determined by GPS gear.

Then NEST operatives, dressed in plain clothes, move in on the ground. Truck-mounted radiation detectors are sometimes used to “read” buildings and vehicles for suspect emissions. When the NEST team gets close to a suspect radiation source, they close in with handheld detectors. Law-enforcement agents then secure the area before turning it over to NEST scientists who are trained to identify, disarm, neutralize, and transport nuclear weapons.